Add GenericDistributionFunction

[Mateasek]

The recent addition of the Function6D framework allows us to add a generic distribution function defined through the Function6D, Function3D and VectorFunction3D frameworks.

I propose making a new distribution function called GenericDistributionFunction which would be defined by Function6D giving the phase space particle density (evaluate), Function3D giving the density, and effective_temperature and VectorFunction3D giving the bulk_velocity.

This would allow users to experiment to a great extent without the need to touch cython code.